To identify antigens in malignant melanoma cells recognized by the free antibodies in human sera and those hidden in soluble immune complexes, we prepared electrophoretic transblots of tumor cell proteins fractionated by electrophoresis in polyacrylamide gels in the presence of detergent. These new methods facilitated the identification of a 30 kilodalton antigen in extracts of the CAR-2 malignant melanoma cell that is recognized preferentially by sera of patients with this disease. Additional studies of the monoclonal antibody-defined melanoma-associated 100 kilodalton antigen demonstrated that high levels of this antigen were present in sera of patients with stage III and IV disease. In sera, the 100 kilodalton antigen forms complexes with albumin, not immunoglobulin. Further studies of the circulating complexes in human sera revealed that these are frequently rich in antibodies specific for Fab' fragments of IgG and may be the circulating residue of ongoing idiotype-anti-idiotype interactions within the humoral limb of the immune system. These naturally occurring anti-immunoglobulins were shown to have a suppressive influence on antibody biosynthesis by pokeweed mitogen-stimulated, human peripheral blood lymphocytes. (IS)